Shape memory alloy actuator

ABSTRACT

A linear actuator includes a plurality of sub-modules disposed in adjacent array and adapted to translate reciprocally parallel to a common axis. A plurality of shape memory alloy wires extend generally linearly and parallel to the axis, and are each connected from one end of a sub-module to the opposed end of an adjacent sub-module. The SMA wires are connected in a circuit for ohmic heating that contracts the SMA wires between the sub-modules. The sub-modules are linked by the SMA wires in a serial mechanical connection that combines the constriction stroke displacement of the SMA wires in additive fashion to achieve a long output stroke. Moreover, the sub-modules are assembled in a small volume, resulting in an actuator of minimal size and maximum stroke displacement. The sub-modules may be rods or bars disposed in closely spaced adjacent relationship, or concentric motive elements, with the serial mechanical connection extending from each motive element to the radially inwardly adjacent motive element, whereby the innermost motive element receives the sum of the translational excursions of all the motive elements concentric to the innermost element. The SMA linear actuator includes a restoring spring assembly having a restoring force that decreases with increasing displacement to minimize residual strain in the SMA components. The SMA wires are connected for ohmic heating in various series and parallel circuit arrangements that optimize force output, cycle time, current flow, and ease of connection.

REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation of application Ser. No.09/566,446, filed May 8, 2000, now U.S. Pat. No. 6,326,707, issued Dec.4, 2001, for which priority is claimed.

BACKGROUND OF THE INVENTION

[0002] Linear actuators find widespread applications in industrial,commercial, vehicular, and domestic settings, in uses ranging widelyfrom electric door locks and windshield wipers in automobiles to pinpullers and shutter controllers in mechanical designs. Generallyspeaking, linear actuators comprise solenoid devices in which anelectromagnet is used to translate an armature, and the retraction orextension of the armature is operatively connected in a mechanism toperform useful work. Such devices are commodity items that aremanufactured in many sizes, force/stroke outputs, and AC or DCoperation.

[0003] Despite their widespread adoption, electromagnetic linearactuators have several important drawbacks that require designaccommodations in mechanical systems. Due to the use of electromagnetismas the motive force, these devices necessarily require ferromagneticmaterials to define the armature as well as a magnetic flux circuit tomaximize the stroke force. Such materials are typically dense, and theiruse results in devices that are rather large and heavy, particularly incomparison to their stroke/force output characteristics. Moreover, themultiple turns of wire that comprise an electromagnet, typicallyhundreds or thousands, add another substantial mass to the device.

[0004] Another drawback of electromagnetic linear actuators is also dueto the use of electromagnetism as the driving force. Typically, as thearmature is extended from the electromagnetic, increasing portions ofthe armature are removed from the influence of the electromagneticfield, and the driving force is concomitantly reduced. As a result, theforce versus stroke displacement characteristics of these devicesgenerally exhibit high initial force values that decline rapidly withincrease in stroke displacement. In many mechanisms it is desirable todeliver a constant force linear stroke, and it is necessary to designadditional mechanisms to make use of the negatively slopedforce/displacement characteristic.

[0005] In recent years much interest has been directed toward shapememory alloy (hereinafter, SMA) materials and their potential use inlinear actuators. The most promising material is nickel titanium alloy,known as Nitinol, which, in the form of a wire or bar, delivers a strongcontraction force upon heating above a well-defined transitiontemperature, and which relaxes when cooled. Assuming the Nitinol wire isheated ohmically or by extrinsic means, there is no need for theferromagnetic materials and numerous windings of the prior artelectromagnetic linear actuators, and there is the promise of alightweight linear actuator that delivers a strong actuation force.Moreover, the force versus displacement characteristic of SMA is muchcloser to the ideal constant than comparable electromagnetic devices.

[0006] Despite the great interest in SMA actuators and many forms of SMAactuators known in the prior art, no practical SMA actuator mechanismhas proven to be reliable over a large number of operating cycles. Ithas been found that Nitinol wire requires a restoring force to assistthe material in resuming its quiescent length when its temperature fallsbelow the material's transition temperature. Many prior art SMA actuatordesigns have made use of common spring assemblies, such as helical orleaf springs, to exert the required restoring force. These springassemblies typically deliver a spring force that varies linearly withdisplacement, (F=kx), and the restoring force in most cases is a maximumat maximum stroke. It has been found that the SMA component respondspoorly to this force/displacement characteristic, and the useful life ofthe SMA actuator is severely limited by such a restoring force. Toovercome this problem, prior art designers have attempted to use simpleweights depending from pulleys to exert a constant restoring force onthe SMA component. Although more effective, this expedient results in amechanism that is not easily realized in a small, widely adaptivepackage.

[0007] Another drawback inherent in known SMA materials is therelatively small amount of contraction that is exerted upon heating pastthe transition temperature. The maximum contraction is about 8%, and theuseful contraction for repeated use is about 6%. Thus, to achieve adirect displacement stroke from the SMA component of about one inch, theSMA component must be over sixteen inches long. This material limitationresults in a minimum size that is too large for many applications. Someprior art designs overcome this problem by wrapping the SMA wire aboutone or more pulleys to contain the necessary length within a shorterspace. However, the SMA wire tends to acquire some of the curvature ofthe pulleys as it is repeatedly heated and cooled, and loses too much ofits ability to contract longitudinally. The result is failure after afew number of operating cycles. Other prior art designs employ leverarrangements or the like to amplify the SMA displacement, with aconcomitant reduction in output force.

[0008] It is evident that the prior art has failed to fully exploit thefull potential of shape memory alloy, due to the lack of a mechanismthat capitalizes on the useful material characteristics of SMA.

SUMMARY OF THE INVENTION

[0009] The present invention generally comprises a linear actuator thatemploys a shape memory alloy component to deliver a relatively longstroke displacement and reiterative operation over a large number ofcycles.

[0010] In one aspect, the invention provides a plurality of SMAsub-modules, each capable of displacement upon heating of the respectiveSMA component. The sub-modules are linked in a serial mechanicalconnection that combines the stroke displacement of the sub-modules inadditive fashion to achieve a relatively long output stroke. Moreover,the sub-modules may be assembled in a small volume, resulting in anactuator of minimal size and maximum stroke displacement.

[0011] The sub-modules may be fabricated as rods or bars adapted to bedisposed in closely spaced adjacent relationship, each rod or bar linkedin serial mechanical connection to the adjacent rod or bar.Alternatively, the sub-modules may comprise concentric motive elements,with the serial mechanical connection extending from each motive elementto the radially inwardly adjacent motive element, whereby the innermostmotive element receives the sum of the translational excursions of allthe motive elements concentric to the innermost element. For all thesub-module embodiments, the serial links therebetween are provided byone or more shape memory alloy wires, each wire connected at opposedends of adjacent sub-modules to apply contractile force therebetween.

[0012] In another aspect, the invention provides an SMA linear actuatorassembly employing a spring assembly that is designed to apply arestoring force tailored to optimize the longevity of the SMA component.In one embodiment of the spring assembly, a roller/band spring(hereinafter, rolamite) is connected to the output shaft of the linearactuator assembly. The rolamite spring exerts a restoring forcecharacterized by a decrease in force with increasing displacement, sothat the SMA components are returned to their quiescent form with aminimum of residual strain. In a further embodiment, the spring assemblyis comprised of a bar or rod connected to the output shaft of the SMAactuator assembly and confined in a channel for longitudinal translationtherein. The bar includes shaped cam surfaces extending longitudinallytherealong, and a cam follower extends from the channel and isresiliently biased to engage the cam surfaces. As the bar is translatedby actuation of the SMA linear actuator assembly, the cam followerexerts a restoring force that is a function of the slope of the camsurface and the magnitude of the resilient force on the cam follower. Byappropriate shaping of the cam surface, the assembly exerts on the SMAlinear actuator assembly a restoring force characterized by a decreasein force with increasing displacement, whereby the number of cycles ofoperation is maximized.

[0013] In a further aspect, the invention includes a housing in which aplurality of drive rods are arrayed in generally parallel, adjacentrelationship and supported to translate freely in their longitudinaldirections. One end of each drive rod is connected to the opposed end ofan adjacent drive rod by an SMA wire, defining a series of driveassemblies connected in additive, serially linked chain fashion. At oneend of the chain, the drive assembly is joined by an SMA wire to thehousing, and at the other end of the chain, the housing is provided withan opening through which an actuating rod may extend. Also secured inthe housing is a spring, such as a rolamite roller/band spring, havingone end connected to the housing and the other end connected to theactuator rod. The spring is designed to exert a restoring force having aconstant or negative force versus displacement relationship.

[0014] Each SMA wire is connected in an electrical circuit, in one ofseveral arrangements of series or parallel connections, so that ohmicheating may be employed to heat the SMA wires beyond their phasetransition temperature. In the chain-connected series of SMA driveassemblies, the resulting contraction of the SMA wires is cumulative andadditive, and the actuating rod is driven to extend from the housingwith a high force output. When the current in the circuit is terminated,the SMA wires cool below the transition temperature, and the springrestores the SMA wires to their quiescent length by urging the actuatingrod to translate retrograde and (through the chained connection ofassemblies) to apply sufficient tension to re-extend all the SMA wires.

[0015] It may be appreciated that the SMA wires remain in substantiallylinear dispositions throughout the contraction/extension cycle, so thatflex-induced stresses are avoided. To assist in heat removal for highpower applications, the housing may be filled with oil or other thermalabsorber, which may be cooled passively or actively. To deliveradditional force, two or more SMA wires may be connected between thedrive assemblies, rather than one wire. To provide enhanced actuationand retraction times, the SMA wires may be thinner.

[0016] Although the invention is described with reference to the shapememory component comprising a wire formed of Nitinol, it is intended toencompass any shape memory material in any form that is consonant withthe structure and concept of the invention.

BRIEF DESCRIPTION OF THE DRAWING

[0017]FIG. 1 is a schematic mechanical diagram depicting the fundamentalcomponents of the shape memory alloy actuator of the present invention.

[0018]FIG. 2 is a cross-sectional elevation of one embodiment of theshape memory alloy actuator of the present invention.

[0019]FIG. 3 is a cross-sectional end view of a negative force constantrolling band spring assembly of the shape memory alloy actuator of thepresent invention.

[0020]FIG. 4 is a plan view of one embodiment of the band spring of therolling band spring assembly depicted in FIG. 3.

[0021]FIG. 5 is a partially cutaway side elevation showing a furtherembodiment of the shape memory alloy actuator of the present invention.

[0022]FIG. 6 is a schematic view of a further embodiment of a negativeforce constant spring assembly of the shape memory alloy actuator of thepresent invention.

[0023]FIG. 7 is a graph depicting force versus displacement fordifferent spring assemblies.

[0024]FIG. 8 is a perspective view of a further embodiment of the shapememory alloy actuator of the present invention.

[0025]FIG. 9 is a top view of the embodiment of the actuator inventiondepicted in FIG. 8.

[0026]FIG. 10 is a side elevation of the actuator invention depicted inFIGS. 8 and 9.

[0027]FIG. 11 is a top view of the assembled drive rods of the shapememory alloy actuator depicted in FIGS. 8-10.

[0028]FIG. 12 is an exploded view of the drive rod assembly of the shapememory alloy actuator depicted in FIGS. 8-11.

[0029]FIG. 13 is an exploded view of the drive rod assembly of the shapememory alloy actuator depicted in FIGS. 8-12, with the drive rods in anextended disposition.

[0030]FIG. 14 is a partial perspective view of a drive rod connection toa shape memory alloy wire, in accordance with the present invention.

[0031]FIG. 15 is a perspective view of a further embodiment of a shapememory alloy actuator employing the drive rod connection assembly shownin FIG. 14.

[0032]FIG. 16 is a cross-sectional end view of a further embodiment of ashape memory alloy actuator of the present invention.

[0033]FIG. 17 is a perspective view of one motive element of the shapememory alloy actuator shown in FIG. 16.

[0034]FIG. 18 is a schematic depiction of one series electrical circuitarrangement for heating the SMA wires of the shape memory alloy actuatorof the invention.

[0035]FIG. 19 is a schematic depiction of a series electrical circuitarrangement for heating paired SMA wires of the shape memory alloyactuator of the invention.

[0036]FIG. 20 is a schematic depiction of another series electricalcircuit arrangement for heating paired SMA wires of the shape memoryalloy actuator of the invention.

[0037]FIG. 21 is a schematic depiction of another series electricalcircuit arrangement for heating paired parallel SMA wires of the shapememory alloy actuator of the invention.

[0038]FIG. 22 is a perspective view of another embodiment of a shapememory alloy actuator employing the drive rod connection assembly shownin FIG. 14.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0039] The present invention generally comprises a linear actuator thatemploys at least one shape memory alloy component as the drivingelement. The invention provides relatively long stroke displacement withhigh force, and delivers reiterative operation over a large number ofcycles.

[0040] With regard to FIG. 1, one significant aspect of the invention isthe provision of a plurality of stages or sub-modules 31A-31D that formthe linear actuator motor 30. Each sub-module 31 includes alongitudinally extending rod 32, and end brackets 33 and 34 secured tothe lower end and upper end of the rod 32, respectively. The sub-modules31 are arranged to translate reciprocally in the longitudinal direction.Note that the brackets 33 and 34 are generally parallel and extend inopposed lateral directions. A SMA wire 36A extends from the lowerbracket 33A to an anchor point 37, SMA wires 36B extends from the lowerbracket 33B of sub-module 31B to the upper bracket 34A of sub-module31A, and SMA wires 36C and 36D join sub-modules B to C, and C to D, tocomplete a serial chain connection. The SMA wires 36A-36D are fabricatedto undergo a phase transition upon heating to a predeterminedtemperature to contract approximately 4%-8%. The contractile force andexcursion of each SMA wire, represented by arrows A-D, is appliedbetween the sub-modules 31A-31D, each pulling on the next adjacent one,whereby the contractile excursion of each SMA wire 36A-36D is combinedadditively. Thus the sub-module 31D undergoes the greatest translationwhen all SMA wires contract, as labeled in FIG. 1 as total displacement(stroke). Indeed, the effective length of SMA wire in the mechanism issubstantially equal to the sum of the lengths of all the SMA wires36A-36D. This effective length is achieved in a compact mechanism,without resort to pulleys or other bending of the SMA wires.

[0041] The longitudinal rod 32D may be provided with an extended distalend 38 to facilitate delivering the output of the actuator 30 to operatea mechanism or perform other useful work. The SMA wires may be heated byconnecting them in an electrical circuit that directs a current throughall the SMA wires for ohmic heating. The circuit may extend from anegative terminal to bracket 33D, and thence through SMA wire 36D to theadjacent sub-module 31C, and so on to a positive connection at anchorpoint 37. In this series connection all wires 36 are heated at the sametime and, due to the same current passing through all wires 36, to thesame extent.

[0042] The linear actuator described thus far with respect to FIG. 1will exhibit a limited useful life (one or a few cycles of contractionand extension), due to the fact that SMA wire will not relax fully whencooled below the phase transition temperature, unless a restoring forceis applied in the extension direction. To provide a restoring force, aspring 39 is connected at one end to the bracket 34D of sub-module 31D,and the other end is secured to a fixed structural point. The spring 39is arranged to be extended by outward movement of the bracket 34D, thusundergoing extension that increases as the wires 36 contract. When thewires are cooled and contract, the spring restoring force applied to thebracket 34D is applied equally through the linked sub-modules 31 to allthe SMA wires 36. This restoring force aids the SMA wires in returningsubstantially fully to their original length, thus greatly lengtheningthe useful life of the mechanism 30. Preferred embodiments of the spring39 are described in the following specification, although standard formsof coil, leaf, or elastomer springs will suffice for a limited usefullife of the mechanism 30.

[0043] With regard to FIG. 2, the invention may provide a block-likehousing 41 for securing the sub-modules 31 in a compact assembly. Thehousing includes a plurality of passages 42 extending therethrough ingenerally parallel arrangement to permit the longitudinal rods 32A-32Dto extend therethrough. Likewise, a plurality of passages 43 extendparallel and interspersed with the passages 42, to receive the SMA wires36A-36D therethrough. The passages 42 are dimensioned to permit freelytranslating motion without any significant lateral movement, and thepassages 43 are dimensioned to receive the SMA wires with clearance toeliminate contact. The array of passages 42 and 43 is laid out to acceptthe sub-modules 31A-31D in serial linked fashion, as described above,and this layout may be in a linear arrangement or in a curved plane thatcontains all the axes of the passages 42, further foreshortening theouter dimensions of the housing 41.

[0044] With regard to FIG. 5, a further embodiment of the inventioncomprises a linear actuator 51 having an outer shell-like housing 52defined by front, rear, top, and bottom walls 53-56, respectively, in atrapezoidal configuration, and side walls 57 (only one shown in thecutaway view) extending therebetween to form a closed interior space. Aplurality of track elements 58 are supported on both side walls 57 inparallel arrays that define slots extending longitudinally in aparallel, vertically spaced arrangement. A plurality of drive bars 59are provided, each supported in one of the slots defined by the trackelements 58 and received therein in freely translating fashion in theirlongitudinal direction. The drive bars 59 are disposed in a verticallystacked array, and may extend distally or retract proximally along theslots in which they are supported.

[0045] A plurality of SMA wires 61 is provided, each extending betweenand connected to the proximal end of one drive bar 59 and the distal endof the vertically superjacent drive bar. At the top of the verticallystacked array of drive bars, the SMA wire 61 is connected at its distalend to an anchor point 62. At the bottom of the vertically stacked arraythe drive bar 59′ is provided with an elongated distal end that isaligned with a window 63 in end wall 53, through which it may extend.The SMA wires 61 may be heated to a temperature above the phasetransition temperature to contract the wires 61. (Electrical wireconnections are not shown for simplification of the drawing.) Each drivebar 59 is advanced incrementally, as shown by the arrow at the distalend of each bar 59, and, since each wire 61 is anchored in thesuperjacent moving bar, the incremental translation of each bar isapplied to the subjacent bar. Consequently, the lowermost bar 59′undergoes the greatest longitudinal translation, extending through theopening 63 to perform useful work.

[0046] The SMA wires undergo a contraction of approximately 4%-8%. Inthe embodiment of FIG. 5, the configuration of the SMA wires determinesthat the contractile force is exerted substantially along thelongitudinal directions of the drive bars 59, and that the angle of theforce vector does not change appreciably between the contracted andextended states of the wires 61.

[0047] A spring assembly 64 is disposed below the lowermost drive bar59′, and is attached thereto to apply a restoring force to bar 59′ andthus to all the SMA wires 61. The spring assembly 64 comprises arolamite spring, known in the prior art and described fully in SandiaLaboratory Report no. SC-RR-67-656, and available from the Clearinghousefor Federal Scientific and Technical Information of the National Bureauof Standards. Briefly, the spring consists of a pair of rollers 65retained within chamber 66, and a band spring 67 that is passed aboutboth of the rollers 65 in an S configuration. The band spring 67includes a tongue 68 extending therefrom through opening 69 and securedto the drive bar 59′. The rolamite spring tongue exerts a specified,engineered restoring force on the bar 59′ to assure that all the SMAwires 61 return to their fully extended disposition when the wires 61are cooled below their shape memory transition temperature.

[0048] As shown in greater detail in FIGS. 3 and 4, the band spring 67preferably is provided with an internal cutout 71 in an extended Uconfiguration to define the longitudinally extending tongue 68. Thechamber 66 is defined by upper and lower walls 72 and 73, respectively,to constrain vertical movement of the rollers. Side walls 74 (only oneshown) join the upper and lower walls, and constrain lateral movement ofthe rollers 65, so that the rollers 65 may move only longitudinally inthe chamber 66. The band spring 67 is secured at a proximal end to theinner surface of the lower wall 72, and is passed about the two rollers65 in an S configuration, as evident in FIG. 3. The distal end of theband spring 67 is secured to the inner surface of the upper wall 73, andthe tongue 68 diverges from the S configuration to extend through thewindow 69 to join the drive bar 59′. As the tongue 68 extends from theopening 69 it pulls the band spring 67 distally, causing the rollers toroll on their respective portions of the band spring as they translatedistally. The spring return force exerted on the tongue 68 is directlyrelated to the difference between the energy liberated as portions ofthe band unbend versus the energy required to bend other portions of theband when the two rollers translate longitudinally. By selectivelyvarying the width of the band spring 67, or selectively varying thewidth of the cutout 71, it is readily possible to generate a springreturn force that follows a predictable mathematical function.

[0049] As depicted graphically in FIG. 7, a typical prior art helicalspring or leaf spring develops a restoring force F that varies generallylinearly with displacement x, or, F=−kx. For a rolamite spring, thefunction that relates spring return force with displacement may differsignificantly from a typical coil spring or leaf spring. In particular,for restoring the SMA linear actuator mechanisms described herein, ithas been found that the optimal force for restoring the SMA wires tofull extension is one having a negative force constant; i.e., therestoring force decreases as extension of the spring increases. Thisforce characteristic preserves the shape memory effect to the maximumextent, and results in a useful working life (in terms of total numberof cycles of operation) in the same range as typical prior art linearactuators.

[0050] In other words, the slope of the graph representing the springfunction exhibits a negative slope in at least a portion of the springexcursion. If the negative slope is constant, the graph will be linearand parallel to line A of FIG. 7. The negative slope may change atdifferent spring sections, producing a graph B comprised of severalcontiguous linear segments. Or the negative slope may vary continuously,producing a smoothly curved graph of the spring function, as representedby graphs C and D. (The band spring may also be fashioned to definepositive slope areas, discontinuous spring functions, detent and dwellportions, neutral spring force, and the like, as required to providethese desired mechanical functions.)

[0051] It should be noted that the contractile force of the SMA wirephase transition is substantially constant as contraction takes place.As a result, the force delivered by the linear actuators describedherein is substantially constant throughout the outward excursion of theactuator. This desirable characteristic is in marked contrast to typicalsolenoid actuators, which produce maximum force at initial actuation andtaper off significantly as translation progresses.

[0052] With regard to FIG. 6, a further embodiment of a return springhaving a having a negative force constant; i.e., the restoring forcedecreases as extension of the spring increases. A bar or similar movingelement 76 is disposed in a channel 77 and is constrained to translatelongitudinally therein, as shown by arrow L. The element 76 includes aside surface 78 defined by contiguous surface portions 78A-78C thatcomprise a camming surface. A cam follower 79 is comprised of atelescoping mounting for a roller and a spring for urging the roller toengage the camming surfaces 78A-78C. The roller is mounted to roll alongthe camming surfaces as they translate along the channel in thelongitudinal direction. On an opposed side of the element 76, arectangular cutout portion 81 defines a linear, longitudinal surface 82engaged by a cam follower 79′. The cam follower 79′ is provided to applya lateral force to the element 76 to counterbalance the lateral forceimparted by cam follower 79, so that the element 76 will avoid becomingjammed in the channel 77.

[0053] It may be appreciated that the resilient force impinging camfollower 79 into camming surfaces 78 is resolved by classical mechanicstechniques into vector forces exerted longitudinally and laterally onthe element 76. The lateral forces are offset by the follower 79′ andthe channel constraints, so that the longitudinal force component urgesthe element 76 to translate longitudinally, thereby constituting arestoring force. For example, as the element 76 translates distally (tothe right in FIG. 6), the cam follower 79 encounters the steeply angledcam surface portion 78B, and exerts a strong, substantially constantlongitudinal restoring force. When the cam follower 79 progresses andimpinges on the camming surface portion 78A, the restoring force isdecreased to a lower constant due to the shallower slope. (The surface78 may comprise any number of segments, curves, or other features.) Asthe element translates proximally under the urging of the cam follower79, the portion 78C acts as a stop to prevent further proximaltranslation. The spring assembly is capable of generating any desiredrestoring force function.

[0054] With regard to FIGS. 8-13, a further embodiment of the linearactuator of the present invention includes a housing 91 having agenerally rectangular exterior and defining a rectangular interior space94 extending longitudinally therein. A bottom plate 93 and a top plate92 close the opposed ends of the space 94, and the output plunger 95 ofthe actuator extends longitudinally through the central hole 97 of thetop plate. Within the space 94 a matrix of drive rods 96 is disposed inclosely packed array, the dimensions of the space 94 and the closespacing of the rods 96 constraining the rods 96 to be translatable onlyin the longitudinal direction. The rods 96 are formed as rectangularparallelepipeds, with each longitudinally extending rectangular surfaceof each rod being adapted to receive and secure one SMA wire, asdetailed below. This construction enables any two rods 96 in the matrixto be connected together, end to opposite end, whether they arelaterally or vertically adjacent (as viewed in FIG. 11. The matrix alsoincludes a spring housing 98 occupying the space of one drive rod 96, asshown in FIGS. 11 and 12, and enclosing any form of return springdescribed herein. The drive rod 96G at the center of the matrix supportsthe output plunger 95, and is connected to the spring within the housing98, so that the spring applies a restoring force to all the SMA wiressufficient to restore the wires to their original length when cooled.

[0055] Drive rod 96A may be connected at its lower end to an SMA wirethat is connected at its upper end to the housing 91. The upper end ofrod 96A is connected to an SMA wire that extends to the lower end of rod96B. Likewise, rod 96B is connected to rod 96C, and so forth to rods96D-96G, which supports the output plunger 95. When all the SMA wiresare actuated, the drive bars 96A-96G extend in additive fashion, asshown in FIG. 13, to push the plunger 95 longitudinally with a strong,constant force. Although the array of drive bars 96 is depicted as a[3×3] matrix, the arrangement may take the general form of any [M×N]array.

[0056] With regard to FIG. 14, the drive bars 96 include at least one ofthe longitudinally extending channels 101-104, each disposed in one ofthe four longitudinally extending rectangular faces of theparallelepiped configuration. Each channel 101-104 is dimensioned toreceive and secure one SMA wire 106. The wire 106 is provided with amounting die 107 crimped to each end thereof, and a retaining pin 108extends across the end of the channel to pinch the die 107 between thepin 108 and the sloped bottom surface at the end of each channel. Theopposed ends of each pin 108 are secured in a passageway 109 extendingfrom opposed sides of the bar 96 and intersecting the channel 101. Theprovision of the channels 101-104 on each face of the bar 96 enables theconnection of any bar 96 to any adjacent bar 96, whether verticallystacked or laterally adjacent. Each channel 101-104 may be prepared asdescribed with reference to channel 101 to effect interconnection of theadjacent bars 96. The channels 101-104 enable the wires 106 to extendbetween the opposite ends of adjacent impinging bars 96 without anycontact or mechanical interference imparted to the wires by the bars.

[0057] The crimped die 107 is formed of a conductive metal, and theengagement of the pin 109 enables electrical connection to the wires 106by the simple expedient of securing the connecting wires to the outerends of the pins 109.

[0058] With regard to FIG. 15, a further embodiment of the linearactuator of the invention makes use of a drive bar 96 as shown anddescribed with reference to FIG. 14. In this embodiment the bars 96 areprovided with top and bottom channels 102 and 103, and are verticallystacked to be linked in serial, additive fashion as describedpreviously. The vertical stacks (two shown, but any number is possible)are supported by side panels 111 and 112, the side panels supporting atleast one circuit board 113 that controls the application of current tothe SMA wires of the vertical arrays. Conductors 114 extend from eachcircuit board to the mounting pins 108 of the adjacent drive barvertical stack to complete circuits through the SMA wires.Alternatively, the circuit board may provide brush contacts that engagesliding contact pads placed on the drive bars 96. In this embodiment thetopmost drive bar undergoes the additive translation of all thesubjacent bars, as described previously.

[0059] A further embodiment of the return spring 39 is shown in FIG. 22with reference to the embodiment depicted in FIG. 15. However, thisspring construction may be employed with any of the linear actuatorembodiments described herein. Drive bar 96′ at the upper end of thevertically stacked array of drive bars 96 undergoes the maximumlongitudinal displacement, and operates the output plunger (not shown)of the array. A base plate 121 joins the side panels 111 and 112 belowthe array of drive bars. A deflection pin 122 extends laterallyoutwardly from drive bar 96′, and an elastically deformable beam 123extends upwardly from the base plate 121 adjacent to the verticallystacked array, with the upper end of the beam disposed to impinge on thedeflection pin 122 when the actuator is retracted. When the SMA wiresare heated and contract, the longitudinal translation of bar 96′ drivesthe deflection pin 122 to bend the beam 123 elastically, therebyexerting a restoring force on the bar 96′ and on the array of drive barsconnected thereto. The beam 123 may be shaped with a non-uniformcross-section, or provided with other aspects that provide a returnforce function that approximates the spring functions A-D of FIG. 7sufficiently closely to provide full return of the SMA wires to theirelongated state, and also a high number of repetition cycles.

[0060] With reference to FIGS. 16 and 17, a further embodiment of thelinear actuator of the invention includes a plurality of drive module126, each comprising a tubular member of rectangular cross-section,although circular and polygonal cross-sections are equally usable. Thedrive modules 126 are dimensioned to be disposed in concentric, nestedfashion with sufficient clearance for telescoping translationtherebetween. Each drive module 126 includes a plurality oflongitudinally extending projections 127, each projection 127 extendingfrom a medial end portion of one side of the respective drive module126, as shown in FIG. 17. (For a cylindrical tubular array, theprojections are spaced at equal angles about the periphery of the end ofeach drive module.)

[0061] Each side of each drive module 126 is provided withlongitudinally extending channels 101 and 103 on the outer and innersurfaces, respectively, the channels being constructed as described withreference to FIG. 14. Each projection 127 supports a mounting pin 108received in aligned holes 109 to retain the crimped die 107 of an SMAwire 106, as described previously. The inner channel 103 providesclearance for the SMA wire of the nested drive module disposedconcentrically within. The number of SMA wires used may vary; in theembodiment shown in FIG. 17, at least two SMA wires 106 are used atradially opposed sides of the nested modules to provide balancedcontractile forces that resist binding of the telescoping elements. FourSMA wires per module may be used, one secured to each projection 127, toprovide maximum contractile force and maximum force to the actuatingplunger. A return spring assembly, of any construction discussed herein,may be placed within the inner cavity of the innermost concentricelement 126 and connected between the innermost and outermost elements126.

[0062] The SMA wires 106 of any contractile array described herein maybe connected for ohmic heating by any of the circuit arrangementsdepicted in FIGS. 18-21. In these Figures, each drive element 140 mayrepresent any of the drive bars or drive modules 32, 59, 96, or 126described previously. Single SMA wires 141 are connected at like ends ofthe elements 140 by extendable wires (or sliding brush contacts) 146 toform a continuous series circuit that includes all of the SMA wires 141.The moving end of the series circuit is connected to lead wire 143 andthe other end, which is fixed in anchor point 142, is connected to lead144 of the current source that actuates the array. This circuitarrangement assures that all wires carry the same current.

[0063] With regard to FIG. 19, a pair of SMA wires 141 are extendedbetween each pair of drive elements 140, thereby multiplying the forceoutput. This arrangement is depicted in the embodiments of FIGS. 15-17and 22, although all embodiments may support multi-wire arrangements.The paired SMA wires are electrically isolated each from the other, andextendable wires 147 (or sliding brush contacts) are secured to the likeends of the paired SMA wires, so that each pair of SMA wires isconnected in series. The series pairs are likewise connected in a serieselectrical circuit by extendable wires 148, with lead wires 143 and 144extending from the same end of the array. (It may be appreciated thatthe number of wires extending between adjacent drive stages may be anyinteger number other than two.) This arrangement provides multipliedforce output using a series circuit to actuate the wires.

[0064] With regard to FIG. 20, a further embodiment of the electricalactuating circuit of the invention includes paired SMA wires 141extending between adjacent drive elements 140, the paired wires beingelectrically isolated each from the other. Extendable wires 151interconnect the SMA wires so that each SMA wire of each pair isconnected in series with one of the SMA wires of the adjacent driveelement. Thus the circuit is comprised of two series branches thatextend from the anchor point 142 to the proximal end of the output driveelement 140, where they are bridged by connection 152. This connectionarrangement provides multiplied output force and, most notably, bothleads 143 and 144 from the power circuit are connected at the fixedanchor point 142, so that the leads are not connected to a movingobject.

[0065] Another embodiment of the actuating circuit, depicted in FIG. 21,also makes use of paired SMA wires 141 extending between adjacent driveelements 140. In this arrangement each pair of SMA wires is connected inparallel, and the paralleled wires are connected by extendable wires 154in a series circuit. Lead 144 connects to the anchor point of the array,and lead 143 is connected at the proximal end of the output driveelement 140. This circuit arrangement provides the multiplied forceoutput from a current draw that is double that of the previousembodiments.

[0066] Previous embodiments, such as those shown in FIGS. 15 and 22,depict electrical power connections from the circuit board to each drivebar assembly. This feature permits any of the connection schemesdescribed above, and also permits direct connection to each SMA wire forindividual actuation thereof. Thus actuation of the SMA wires may becarried out simultaneously, or staged sequentially in individual orgrouped actuations.

[0067] It is noted that there is a direct correlation between thediameter of the SMA wires and the recovery (relaxation) time of themechanisms described herein. That is, finer wire yields shorter recoverytimes. Multiple fine wires between adjacent drive elements may be moreadvantageous (in terms of actuation and recovery times) than a single,heavier gauge SMA wire, while producing approximately the same thrust.All embodiments of the invention have the explicit or implicitcapability to use multiple SMA wires between adjacent stages of themechanism.

[0068] In the embodiments of the linear actuator described herein inwhich the drive elements are enclosed in a housing, the housing may befilled with a liquid such as oil, ethylene glycol anti-freeze, orsimilar liquid that is lubricious and heat conducting. Such fluidenhances the speed of cooling of the SMA wires by a factor of one or twoorders of magnitude, thereby increasing the rate of contraction of theSMA wires and enabling a far faster actuation and cycle rate for theassemblies. The extension and retraction of the drive elements aids incirculating the fluid for cooling purposes. The fluid may be pumpedthrough the housing for maximum cooling effect in high duty cyclesituations.

[0069] Although the invention is described with reference to the shapememory component comprising a wire formed of Nitinol, it is intended toencompass any shape memory material in any form that is consonant withthe structural and functional concepts of the invention.

[0070] The foregoing description of the preferred embodiments of theinvention has been presented for purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise form disclosed, and many modifications andvariations are possible in light of the above teaching without deviatingfrom the spirit and the scope of the invention. The embodimentsdescribed are selected to best explain the principles of the inventionand its practical application to thereby enable others skilled in theart to best utilize the invention in various embodiments and withvarious modifications as suited to the particular purpose contemplated.It is intended that the scope of the invention be defined by the claimsappended hereto.

1. A linear actuator, including: at least one sub-module adapted toundergo reciprocal translation in a first direction; at least one shapememory component, extending generally in said first direction; means forheating said at least one shape memory component beyond the memorytransition temperature to contract said shape memory component and urgesaid at least one sub-module to translate in said first direction andundergoing a stroke displacement.
 2. The linear actuator of claim 1,wherein said means for heating includes an electrical circuit connectedto said at least one shape memory components for ohmic heating thereof.3. The linear actuator of claim 1, further including return spring meansfor resiliently opposing said stroke displacement.
 4. The linearactuator of claim 3, wherein said return spring means generates a returnforce versus displacement characteristic that is optimized to relax andextend said at least one shape memory component with minimum residualstrain.
 5. The linear actuator of claim 4, wherein said return springmeans comprises a rolamite spring assembly.
 6. The linear actuator ofclaim 3, further including a fixed anchor point, and said return springmeans is connected between said at least one sub-module and said fixedanchor point.
 7. The linear actuator of claim 1, further including ahousing having interior features impinging on said at least onesub-module to support said sub-module in reciprocally translatingfashion.
 8. The linear actuator of claim 1, further including means forcooling said at least one shape memory component.
 9. The linear actuatorof claim 1, wherein said means for cooling includes a heat-conductingfluid surrounding said at least one shape memory component.
 10. Thelinear actuator of claim 3, wherein said return spring means generates areturn force versus displacement characteristic that is substantiallylinear through a portion of the excursion of said return spring means.11. The linear actuator of claim 3, wherein said spring means comprisesa deflectable beam spring.
 12. The linear actuator of claim 3, whereinsaid spring means comprises a bar adapted for reciprocal translation,said bar including a cam surface, and cam follower means impinging onsaid cam surface to exert a restoring force that is a function of theslope of said cam surface.